1. Field of the Invention
The present invention relates to a digital radio receiver used for digital radio communication, and in particular to a digital radio receiver that can extend the range of carrier offset compensation, to improve demodulation characteristics.
2. Description of the Related Art
In a modem utilizing a multi-value modulation system typified by a 16 QAM modulation system, both of transmitter and receiver (modulator and demodulator) perform modulation and demodulation using carrier signals from local oscillators that are basically on the same local frequency. In this case, a frequency error is caused due to a difference in oscillating frequency accuracy between the transmitter and receiver, a temperature change, a change with time, and so on. The frequency error manifests itself as a frequency offset (carrier offset) of a signal received.
The frequency offset (carrier offset) has become a major problem to be solved not only in the field of next-generation subscriber radio access (FWA: Fixed Wireless Access) systems, but also in keeping with the trend toward higher frequencies, that is, the trend in making frequency (RF: Radio Frequency) of radio communication higher.
The frequency offset manifests itself as a phase rotation with a fixed speed of the received signal after detected. To obtain a correct demodulated signal, the frequency offset needs compensating.
In the case of digital radio communication, there is one typical technique for frequency offset compensation. In this technique, a signal (UW: unique word) known to both the transmitter and the receiver, which may be used for synchronous acquisition or equalizer training, is transmitted by adding the same to transmitted data. On the receiver side, a phase error is detected from the unique word and the received signal, equalized through a PLL circuit or the like, and inputted as a voltage value to a voltage control oscillator as a local oscillator. Thus the local oscillation frequency on the receiver side is controlled to compensate for the frequency offset.
Another conventional technique for frequency offset compensation is a frequency offset compensating circuit using an equalizer.
When a carrier offset is corrected through a carrier offset compensating circuit using an equalizer, a frequency-to-error (BER) characteristic is plotted as shown in FIG. 19. FIG. 19 is a graph showing simulation results of the frequency-to-error (BER) characteristic in the conventional carrier offset compensating circuit.
One of conventional techniques for frequency offset compensation is disclosed in Japanese unexamined patent publication No. 10-98500 laid open on Apr. 14, 1998 and entitled “Automatic Frequency Control Method and Circuit” (Applicant: Kokusai Electric Co. Ltd., Inventor: Hiroki Goto).
In this conventional technique, a delay detector obtains phase differences among a plurality of symbols of a received signal containing a symbol known as a UW and variable data symbols. Based on the output of the delay detector, a discriminator obtains a residual phase-rotation component remaining after a modulation component of each variable data symbol is removed. Then, a phase shifter obtains a frequency offset estimated value, an LPF can do averaging the obtained frequency offset estimated value, and a complex multiplier compensates for the frequency error. The automatic frequency control method and circuit constituted such above can extend the frequency offset compensation range of an AFC used for base band signal processing in a reception/modulation circuit of a multi-value modulation system without deteriorating transmission efficiency.
However, when the conventional carrier offset compensating circuit is used in a TDD communication system, since the local oscillator cannot be controlled in a transmission section, some measures need to be taken such as to keep a control value obtained at the time of completion of reception. In any case, a frequency offset within a range corresponding to that of stability of the local oscillator is unavoidable, which causes a received signal to rotate in a reception section, and hence results in such a problem that the received signal cannot be demodulated properly.
In addition, the carrier offset compensating circuit with the equalizer incorporated therein shows a frequency-to-BER characteristic as shown in FIG. 19, which results in another problem that the carrier offset compensating circuit is limited in its range that can cope with offset frequencies.